1. Field of the Invention
This invention relates generally to brushless DC motors and, more specifically, to a drive circuit for a brushless DC motor.
2. Background Description
Brushless DC motors generally consist of two major stages: a pre-driver stage and an actual driver stage. The actual driver stage may be bipolar or unipolar. A bipolar driver stage consists of four switching devices, e.g., field effect transistors (FETs) or bipolar junction transistors (BJTs), arranged in a full-bridge configuration. The switching devices are driven by complementary pulses generated by the pre-driver stage such that the switching devices that are located diagonally with respect to one another are turned on at the same time. A unipolar driver stage consists of two switching devices arranged in a half-bridge configuration, only one of which is turned on at one time.
The pre-driver stage consists of a discrete integrated circuit (IC) that generates the complementary pulses for the driver stage in response to the output from a Hall sensor. In a fan, the Hall sensor is switched by a magnet included in the turning impeller of the fan. Whenever the impeller of the fan or the motor has made a full revolution, the magnetic field of the impeller magnet changes relative to the position of the Hall sensor so that the output of the Hall sensor switches from one logic state (e.g., a logic low or a logic high) to the complementary logic state. Thus, there is effectively a closed loop from the output of the driver stage back to the pre-driver stage.
The closed loop from the output of the driver stage to the pre-driver stage enables the fan to run essentially self-sufficiently. However, there are some conditions where the fan requires assistance to operate correctly and, very importantly, safely. For example, in a xe2x80x9clocked rotorxe2x80x9d condition, where the fan impeller is stopped for any reason, the fan has to turn itself off in order not to burn out the switching devices in the driver stage. After a predetermined time period of t seconds the fan must determine whether the fan impeller is free to resume rotating. The fan does this by turning on one output of the pre-driver stage and waiting for the impeller to turn. If the impeller does not begin to turn within a predetermined time period, the output of the pre-driver is turned off again. The fan repeats this cycle every t seconds. The timing for the restart cycles is provided by a resistor-capacitor network that is external of the pre-driver IC.
Other features have been implemented to ensure the proper operation of the fan. For example, an alarm can be generated whenever the fan is in a locked rotor condition for greater than a predetermined amount of time. Alternatively, an alarm can be generated whenever the fan is running at speed that is below a certain RPM threshold level. Again, external circuitry is required to generate the alarm under either of these conditions.
Another feature of the fan concerns the tendency of the fan to draw a high inrush current during its startup mode of operation. To counteract this tendency, an external circuit generates a pulse-width modulated (PWM) signal to enable and disable the pre-driver and driver stages in a so-called xe2x80x9cchopping modexe2x80x9d of operation during the startup period. The effect of chopping the inrush current is to inhibit the rate of current flow during the startup period to provide the fan or motor with just enough current to start the fan or motor until the fan or motor is ramped up to full speed.
It is known in the art to provide a discrete IC in conjunction with external circuitry for controlling brushless DC motors including fan motors. U.S. Pat. No. 5,583,404 entitled xe2x80x9cDriver Circuit for Brushless DC Motorsxe2x80x9d and issued to Karwath et al. teaches the general concept of checking for a xe2x80x9clocked rotorxe2x80x9d condition, interrupting the supply of current to the motor for a limited period of time, and activating the alarm function in such circumstances and attempting a re-start after the passage of a predetermined time. Furthermore, it also teaches the use of stepped-up current during a start-up mode of a brushless motor in order to prevent power-on current surges.
U.S. Pat. No. 5,838,127 entitled xe2x80x9cSingle Phase Motor for Laundering Apparatusxe2x80x9d and issued to Young et al. discloses the general concept of utilizing a position sensor for sensing the angular position of the rotatable assembly of a motor relative to the stationary assembly of the motor and generating a control signal as a function of the sensed position to reverse the direction of rotation.
U.S. Pat. No. 4,656,533 entitled xe2x80x9cElectronically Programmable Universal Brushless DC Fan with Integral Tracking and Locked Rotor Protectionxe2x80x9d and issued to Brown discloses the concept of using a commutation sensing device such as a voltage regulator to determine fan speed and to limit the current to a fan during a start-up condition or in a locked rotor condition.
U.S. Pat. No. 5,258,696 entitled xe2x80x9cIC Controller for Brushless DC Motorsxe2x80x9d and issued to Le discloses the concept of using a single IC chip driver for brushless DC fan motors, where the input to the IC driver consists of a signal which represents the rotor""s position with respect to the stator""s windings and which is used by the IC driver to generate commutation commands and commutate power to the motor""s stator windings to drive the motor. Similarly, U.S. Pat. No. 5,350,988 entitled xe2x80x9cDigital Motor Controllerxe2x80x9d and also issued to Le discloses the same concept, wherein the analog position signal of the rotor with respect to the stator is converted to digital data to be processed by the digital controller.
U.S. Pat. No. 5,013,985 entitled xe2x80x9cMicrocomputer with Motor Controller Circuitxe2x80x9d and issued to Itoh et al. (xe2x80x9cthe ""985 Patentxe2x80x9d) suggests use of a microcomputer with CPU, read only memory (ROM) and random access memory RAM in a single chip with a motor controller circuit. However, the ""985 Patent does not suggest use of a microcomputer as the driver circuit for the motor. Instead, the microcomputer provides signals to the motor controller circuit for generating three-phase inverter waveforms. Neither does it suggest any of the previously mentioned features that are used to ensure the proper operation of a brushless DC fan.
U.S. Pat. No. 5,317,244 entitled xe2x80x9cMotor Control Unit Provided with Anti-Burning Devicexe2x80x9d and issued to Ishikura describes a motor controller with the capability to prevent the burning out of the circuits by limiting the time in which current is supplied to the motor if the rotational speed of the motor falls below a predetermined level or if a xe2x80x9clocked rotorxe2x80x9d condition occurs. Similarly, U.S. Pat. No. 5,327,052 entitled xe2x80x9cMethod and Apparatus for Controlling Brushless DC Motorxe2x80x9d and issued to Sakurai et al. describes a method for providing stepped-up current during the start-up mode of a brushless DC motor by reading and evaluating the rotor position and increasing the drive current to the DC motor at a predetermined rate until the rotor is rotated. Likewise, xe2x80x9cchopping modexe2x80x9d operation of a brushless and sensorless DC motor is described in U.S. Pat. No. 5,350,984 entitled xe2x80x9cMethod and Apparatus for Starting a Brushless DC Motorxe2x80x9d and issued to Carobolante et al.
There are significant disadvantages associated with implementing these features using a discrete IC in conjunction with external circuitry. In using a discrete IC, the designer is constrained by the parameters of the particular discrete IC and, to the extent that the discrete IC provides the capability to change the restart timing interval for the locked rotor condition, there exists a limited range of flexibility in changing this restart timing interval. A customer may desire the fan to generate data relating to temperature conditions, speed, and current consumption. The signals generated by the fan may need to change to communicate with the customer""s interface, but a discrete IC used with external circuitry cannot accommodate changes in the data signals generated by the fan. The customer may also want to be able to upgrade his fan to include subsequently developed features and improvements. With a discrete IC driver, the customer is limited to the features provided with the purchased configuration of the fan, and the customer must purchase a new fan to obtain the benefit of any subsequently developed features or improvements.
Another significant drawback associated with implementing these features using a discrete IC in conjunction with external circuitry is the complication and inefficiency involved in manufacturing fans and motors according to a variety of different customer requirements. A manufacturer of fans and motors may need to be able to support a multitude of different driver configurations. For example, not all discrete IC drivers offer similar features, and external circuitry is needed to provide the features that are not provided by the discrete IC. Furthermore, each customer may not need all of the features provided by a particular discrete IC, and even if each customer does need all of the features provided by a particular discrete IC, the parameters of those features may vary from customer to customer. As a result, a multiplicity of physical configurations for the driver stage are inevitable when a discrete IC is used.
Accordingly, there is a need in the art to replace the multitude of different discrete driver configurations for a brushless DC motor with a single driver configuration that is adaptable to provide a customer with a customized combination of features for a brushless DC motor.
The present invention is directed to a system and method that satisfies the need for a single driver configuration that is adaptable to provide a customer with a customized combination of features for a brushless DC motor.
According to an embodiment of the present invention, a drive circuit for a brushless DC motor that controls at least one operation feature of said motor, the at least one feature utilizing at least one parameter that defines the desired operation of the feature, comprises: a driver stage for providing a current to a stator coil; and means coupled to said driver stage for varying said at least one parameter. The means for varying may vary the parameter or parameters without changing the physical configuration of the brushless DC motor. The means for varying may include input means for inputting said at least one parameter. The means for varying said parameters may comprise a microcontroller that executes software program instructions to control the operation features.
One operation feature may comprise limiting the inrush current drawn by said brushless DC motor upon start-up of said motor, and the parameters may comprise a threshold reference value for said inrush current, or a compare value from which a voltage representing said inrush current is subtracted.
Another operation feature may comprise speed control of said brushless DC motor using an analog voltage, and the parameter may comprise a threshold reference value for the speed of said motor.
Yet another operation feature may comprise generating an alarm signal, and the parameters may comprise a threshold level for the speed of said DC motor, a value stored in a counter, or a value indicating a time for generating said alarm.
A further operation feature may comprise determining a locked rotor condition, and the parameter may comprise a value stored in a counter.
An even further operation feature may comprise generating an alarm in response to the detection of a locked rotor condition, and the parameter may comprise a value stored in a counter.
Another operation feature may comprise restarting said motor following the detection of a locked rotor condition, and the parameter may comprise a value stored in a counter.
According to another embodiment of the present invention, a drive circuit for a brushless DC motor that controls at least one of a number of possible operation features of said motor comprises: a driver stage for providing a current for a stator coil; and means for varying which of said number of possible operation features of said motor are controlled.
The possible operation features of said motor may comprise: speed control of said motor using a pulse-width modulated signal; speed control of said motor using an analog voltage; limiting inrush current drawn by said motor upon start-up of said motor; generating an alarm signal if the speed of said motor is below a threshold level; detecting a locked rotor condition; generating an alarm in response to the detection of a locked rotor condition; and restarting said motor following the detection of a locked rotor condition.